One method to measure the BAO spectrum at higher redshift is to directly measure the neutral hydrogen density associated with galaxies by using the 21 cm emission line. This probe is particularly attractive because of the omnipresence of neutral Hydrogen at all redshifts.
Since the characteristic scale of the BAO is very large, there is no need to detect individual galaxies at high resolution, and instead a measurement of the power spectrum of neutral hydrogen density on large scales can completely capture the BAO structure.
This technique is called intensity mapping [1]}.
Galaxies are observed collectively via low spatial resolution measurements of redshifted 21 cm lines of neutral Hydrogen, which also has the potential to make 3D intensity mapping more efficient as compared to optical galaxy surveys.
